The invention is directed to a process for the pyrogenic production of a finely divided oxide of a metal and/or metalloid in which there is employed as the starting material a vaporizable halogen compound of a metal and/or metalloid and in which the elemental halogen formed as by product during the formation of the oxide of the metal and/or metalloid during the cooling of the reaction product is reacted with additionally supplied gaseous hydrogen below the reaction temperature of the oxyhydrogen gas mixture, consisting of the additionally supplied gaseous hydrogen and the oxygen contained in the reaction gases, to form hydrogen halide.
In the pyrogenic production of finely divided oxides of a metal and/or a metalloid there are supplied either separately or in admixture to a burned volatile or vaporizable compounds of the metal and/or metalloid together with combustible or water vapor forming gases.
In connection therewith the combustible gas and the oxygen containing gas are supplied in such a ratio that guarantees a complete burning of the combustible gas and also the hydrolysis of the vaporizable compound of the metal and/or metalloid.
If there are employed as starting materials inorganic or organic halogen compounds of the metals and/or metalloids then the metal oxide and/or metalloid oxide deposits together with a hydrogen halide containing waste gas which is separated off from the metal and/or metalloid oxide in appropriate separating apparatus. Elemental halogen is formed in a side reaction. According to the conditions for carrying on the reaction for the formation of the metal oxide and/or metalloid oxide there is formed, based on the amount of hydrogen halide formed 6 to 10 weight % of elemental halogen.
It is known to remove the elemental halogen, e.g. chlorine formed from the reaction waste gas in a pyrogenic process for production of oxides of a metal and/or metalloid if the elemental halogen formed is reduced with hydrogen during the cooling off of the reaction product below the reaction temperature of the hydrogen and with the oxygen contained in the reaction waste gas (GERMAN OS 2533925).
In this known process in which SiCl.sub.4 (silicon tetrachloride) is used as the starting material for the production of silicon dioxide, the mixture formed of reaction waste gas and silicon dioxide passes through a temperature drop from 1000.degree. to 200.degree. C. within a relatively long cooling section during the cooling. The additional hydrogen is supplied in a region of the cooling section in which the reaction waste gas still has a temperature of 500.degree. to 700.degree. C. An addition of hydrogen at a temperature above 700.degree. C. is not recommended since a reaction of the hydrogen with the oxygen already occurs in this case. The addition of the hydrogen at a temperature of below 500.degree. C. likewise is not recommended since in this case the raction speed of the reaction of the hydrogen with the elemental chlorine is too slow.
The best results can be produced if the elemental hydrogen is introduced at a temperature between 550.degree. and 630.degree. C.
The exact point of introduction for the elemental hydrogen in the cooling section is load dependent, i.e. with production conditioned changes in flow velocity the place of introduction for the elemental hydrogen in the cooling section must be shifted.
A pipe is used for the introduction of the hydrogen, which pipe corresponds in its length to the diameter of the cooling section. This pipe has two series of bores in its jacket through which bores the hydrogen can be introduced into the reaction waste gases. A cross section of this type of introductory pipe is shown in the FIGURE of the present drawings.
The object of an older application in Germany according to P2849851 is the development of a process for the pyrogenic production of the finely divided oxide of a metal and/or metalloid in which there is employed as starting material a vaporizable halogen compound of a metal and/or metalloid in which the elemental halogen formed during the formation of the oxide of the metal and/or metalloid is reacted to halogen halide during the cooling of the reaction products with additionally introduced gaseous hydrogen below the reaction temperature of the oxyhydrogen gas mixture, consisting of the additionally introduced gaseous hydrogen and the oxygen contained in the reaction waste gas, in which gas the gaseous hydrogen is introduced by means of at least one double jacketed tube into the cooling section, in which case there is additionally led in by means of the same double jacket tube an inert gas which is supplied between the outer and inner jacket of the double jacketed tube, Schmid U.S. application Ser. No. 85,358 filed Oct. 16, 1979 corresponds to German application p2849851.